In general, in the production of Al-killed steel, pig iron is decarburized and refined in a converter and tapped into a ladle, and is deoxidized with Al to remove oxygen in molten steel as an oxide, and adjusted to proper composition, and then continuously cast to obtain a cast slab.
In the above deoxidation with Al, means are provided for aggregating and coalescing the oxide through gas agitation or in a RH degassing device to promote the floating of the oxide, but the oxide or alumina inevitably remains in the cast slab.
Such alumina is liable to form clusters. Particularly, when a cluster is caught in a surface layer portion of the cast slab, a surface quality of an extremely-thin steel sheet product such as steel sheet for automobiles, requiring a beautiful appearance, is damaged. In this type of steel sheet, therefore, it is very important to prevent the occurrence of alumina clustesr.
As a means of deoxidizing steel for preventing the formation of alumina clusters, there has hitherto been proposed and disclosed a method of subjecting molten steel (high-carbon steel) to composite deoxidation treatment with an alloying agent consisting of Ca: 10-30%, Al: 2-20%, Mg: 1-15%, Si: 10-60%, Ba: 10-30% and the balance of Fe in, for example, JP-A-51-5224 (method of producing high-cleaned steel).
However, such an alloying agent contains Ba, so that when it is added to molten steel as a deoxidizing agent, there is a problem in the maintenance of the working environment. And also, it contains 10-60% of Si, so that Si remains in molten steel. For this end, it can not be used in the steel sheet for automobiles requiring severe work. Further, when the above alloying agent is used, Ca and Mg having a high steam pressure are added in relatively greater amounts, so that the yield after the addition to molten steel becomes variable. Therefore, the formation of composite inclusions from alumina clusters and the shape controllability of the inclusion become unstable, so that the effect of preventing the formation of alumina clusters becomes variable.
In JP-A-54-116312 (deoxidizing alloy for molten steel) is proposed and disclosed a deoxidizing alloy for molten steel containing 2-10 mol % of one or more elements of Group IIIa (Y, Ce, La and so on) to Al.
However, when deoxidation is carried out by using such a deoxidizing alloy for molten steel, it is effective to prevent the formation of dendritic oxide inclusion and the prevention effect for big clusters is observed, but it is insufficient to reduce clusters having a diameter of about 100 .mu.m, which comes into problems in the steel sheets for automobiles, and also there is a problem that the floating and separating properties of the resulting oxide inclusions are poor.